The present disclosure is related to contrast enhanced medical imaging. In cardiac electrophysiology (EP), trans-catheter radiofrequency (RF) thermal ablations aim for the elimination and/or electrical isolation of the arrhythmia substrate by creating closed loops of thermally ablated tissue. Ablation lesion contiguity and inclusiveness define the procedural long-term success.
Currently, EP procedures are performed under x-ray, electro-anatomical voltage mapping (EAVM) and intra-cardiac echo (ICE) guidance. All these modalities do not provide adequate soft tissue visualization to the electrophysiologist. Hence, there is a substantial and constantly growing interest among the electrophysiologists in using MRI for ablation outcome verification both intra- and post-operatively.
Previously reported studies investigated EP lesion appearance on T1w, T2w (1), delayed enhancement (DE, 2-5) MR imaging, and their combination (6). They proved the feasibility of using MRI techniques for the visualization of fresh and aged ablation lesions. However, these T1w/T2w-based imaging methods provide low contrast between the ablation lesion and surrounding normal tissue. Also, they required rather long imaging times, which handicaps its potential intra-operative applicability. In addition, the current T1w/T2w-based imaging does not have a plausible biological explanation for changes in the MRI appearance of ablation lesions during repetitive imaging post-procedurally (7).
In contrary, contrast agent enhanced (CE) MR imaging has proven to be an accurate and reliable indicator of tissue destruction during thermal ablations (8), corresponding well not only to the histopathological analysis (9), but also to the delivered thermal dose (10, 11). However, delayed contrast enhancement (DE) MR imaging of ablation lesions also requires long waiting (after contrast agent injection) as well as long scanning times, which complicates its intraoperative applications. In addition, ablation lesions' appearance on DE MRI is highly influenced by the time elapsed after contrast agent injection (3) and imaging resolution (7), and thus is difficult to interpret. These drawbacks of the traditional MRI ablation lesion visualization methods are especially pronounced when the goal of the visualization process is not simply to confirm the fact of the existence of ablation lesions per se, but rather to delineate the lesions' borders in order to identify the gaps between them. Indeed, accurate delineation of the outer borders of ablation lesions is the main motivation for intra- and post-operative MR imaging during cardiac ablative procedures.
Dynamic contrast enhancement MRI is an improved method involving the sampling of the process of contrast agent arrival and passage through the tissue per se, and thus does not require long waiting times after contrast agent injection. It has been successfully applied for ablation lesion visualization and characterization (12, 13) as well as for tumor perfusion (14, 15) and viability (16, 17) assessment. Dynamic contrast enhancement MRI is based on the differences in perfusion properties between different tissues or areas of the same tissue—e.g., the lack of perfusion in the tissue areas affected by ablation (due to the occlusion and/or disruption of its vascular structures), which can potentially lead to apoptosis, especially in the myocardium. However, the existing methods of dynamic contrast enhancement image analysis are not suitable for intra- and post-operative imaging during cardiac EP procedures. They rely upon model-based fitting of pixel enhancement curves with certain properties and thus require the whole contrast agent wash-in and wash-out processes to be sampled with high signal-to-noise ratio (SNR). Such requirements not only result in longer MRI scan times, but are also very difficult to satisfy in cardiac MRI restricted by the respiratory and cardiac motion patterns of the imaged anatomy.
Preliminary investigations have indicated (18) that these limitations can be overcome by combining various instantaneous pixel intensity evolution characteristics at each dynamic contrast enhancement sampling instant into cumulative maps, which reflect not only the current signal evolution state of each represented pixel, but also the whole “pre-history” of the pixel, and hence reflect the dynamic contrast enhancement process in general rather its current instantaneous state only. This helps to differentiate between pixels with different contrast enhancement properties, whose differences may be hidden by the image acquisition noise. As a result, such maps are relatively immune to low SNR (which makes them suitable for fast cardiac imaging), and require imaging during only a relatively short time following contrast agent injection to delineate ablation lesion borders without any model-based fitting of curves with special properties anticipated in advance.
Unfortunately, the intra-operative interpretation of dynamic contrast enhancement images, both traditional and cumulative images, poses a substantial challenge to the clinical electrophysiologists, who do not have sufficient amount of MRI expertise and experience. The usage of such techniques requires complex decision-making. Making such analysis and decisions, especially in the midst of performing invasive procedures, requires certain knowledge of MR image formation and acquisition principles, which is potentially beyond the achievable for clinical electrophysiologists at present. This can handicap the acceptance of MRI (especially, the intra-procedural one) as a useful aid during clinical EP procedures.